1. Field of the Invention
The present invention relates generally to a passive safety system of an integral reactor, and more particularly to a passive safety system of an integral reactor, which includes a passive safety injection system (core makeup tank and safety injection tank) and a passive residual heat removal system.
In more detail, the present invention relates to a passive safety system capable of safely protecting a reactor by removing residual heat and maintaining a core water level in a reactor by using natural force such as gas pressure or gravity without active equipment such as a safety injection pump, which requires power supply, or a facility such as a safeguard vessel for a considerable amount of time at which the residual heat emitted from a reactor core is significantly reduced.
2. Description of the Related Art
Different from a typical industrial power plant, a nuclear power plant generates residual heat from a reactor core for a considerable amount of time after a reactor has been shutdown, and an amount of the residual heat is rapidly reduced by lapse of time. Accordingly, the nuclear power plant has various safety facilities to remove the residual heat from the reactor core and to ensure safety upon an accident.
Among several safety facilities, there are a safety injection system and a residual heat removal system as main systems to ensure the integrity of the core. The safety injection system complements a coolant when the coolant of the reactor is lost due to the loss of coolant accident such as the break of a line connected with the reactor, and the residual heat removal system removes sensible heat and residual heat of the reactor after the reactor core has been shutdown.
A passive reactor of a commercial reactor (loop type pressurized water reactor) includes a core makeup tank (high pressure safety injection), a pressurized-type safety injection tank (intermediate pressure safety injection), and an in-containment refueling water storage tank (low pressure safety injection). An active reactor of the commercial reactor includes a high pressure safety injection pump (high pressure safety injection), a pressurized-type safety injection tank (intermediate pressure safety injection), and a low pressure safety injection pump (low pressure safety injection, integrated into high pressure safety injection pump lately.
The safety injection tank applied to the commercial reactor is a device to rapidly supply cooling water into the reactor by using the pressurized nitrogen gas in the safety injection tank when the internal pressure of the reactor is rapidly reduced due to the large loss of coolant accident. In other words, the safety injection tank is designed to cope with the large loss of coolant accident. The safety injection tank is a facility to create the margin of time until coolant is actually injected at a safety injection flow rate from a gravity-driven passive safety injection system or a high pressure safety injection pump, and the safety injection tank is used for a short time (about 1 minute to 4 minutes after actuated).
Accordingly, when the pressure of the reactor is rapidly reduced due to the large loss of coolant accident in the active reactor, the safety injection system of the active reactor is actuated in the sequence of “pressurized-type safety injection tank→high pressure safety injection pump”. When the pressure of the reactor is slowly reduced due to a small loss of coolant accident in the active reactor, the safety injection system of the active reactor is actuated in the sequence of “high pressure safety injection pump→pressurized-type safety injection tank”.
When the large loss of coolant accident or the small loss of coolant accident occur in the passive reactor, the passive safety injection system has the same actuating sequence of “core makeup tank→pressurized-type safety injection tank→in-containment refueling water storage tank” in the two cases. However, since a gravity tank such as the core makeup tank has a low gravitational head, an injection flow rate is low. Accordingly, in the initial stage of the large loss of coolant accident, an injection flow rate of the pressurized-type safety injection tank occupies most parts of a safety injection flow rate. In addition, an automatic depressurization system having a multi-stage structure is installed in a passive reactor (AP1000 in the U.S., loop type reactor) to rapidly lower the pressure of the reactor so that the reactor and the containment make pressure balance in an early stage to smoothly perform gravity safe injection (in-containment refueling water storage tank).
In addition, the passive residual heat removal system removes the sensible heat of the reactor and the residual heat of the core upon an accident. The main cooling water circulating schemes of the passive residual heat removal system include a scheme of directly circulating primary cooling water and a scheme (SMART reactor in Korea) of circulating secondary cooling water by using a steam generator. In addition, a scheme of injecting the primary cooling water into a cooling tank and directly condensing the primary cooling water (AP1000, Nuscale in the U.S.) is partially used.
In addition, emergency core cooling schemes using a safeguard vessel, a pressurized-type safety injection tank, and a passive residual heat removal system in relation to an integral reactor are disclosed in Korean Patent Registration Nos. 10-419194, 10-856501, and 10-813939 issued on Feb. 5, 2004, Aug. 28, 2008, and Mar. 10, 2008, respectively. A reactor having a similar concept, in which a safeguard vessel is applied, has been developed (IRIS, Nuscale in the U.S.).
However, since the safeguard vessel is a pressure vessel that is smaller than a containment building (a containment vessel or a reactor building) but larger than a reactor, the safeguard vessel has a great difficulty in solving problems related to the manufacturing and the transporting of the vessel, the long term of construction works, the integrity of a device installed in the safeguard vessel under a high temperature and high pressure environment upon a loss of coolant accident, and the convenience in refueling and maintenance.
In a core makeup tank, a pressure balance line is connected with a high-temperature line, and an isolation valve is mounted on a safety injection line. The core makeup tank has the same design pressure of the reactor. Accordingly, when the tank is manufactured in large size for the purpose of usage for many hours, the manufacturing cost is greatly increased, and the pressure boundary of the reactor is expanded. In addition, since the safety injection tank is similar to that of the loop type active reactor, the safety injection tank is insufficient for the purpose of the usage for many hours.
In addition, different from the loop type reactor, since an integral reactor fundamentally eliminates a large loss of coolant accident, the reactor is maintained under the high pressure for many hours when the loss of coolant accident occurs. Accordingly, the integral reactor has a difficulty in injecting external cooling water into the integral reactor by gravity without increasing the external pressure of the reactor (pressure balance) through a safeguard vessel.
Further, in a passive residual heat removal system to cool a primary system of the reactor through a direct circulation scheme, the pressure boundary of the primary system is expanded to a condensation heat exchanger in actuation, so that the condensation heat exchanger must be installed in the containment. Accordingly, the containment must be designed to serve as an ultimate heat sink in the final stage. As described above, since the pressurized-type safety injection tank applied to the commercial reactor must be designed at higher pressure, the manufacturing cost of the pressurized-type safety injection tank is greatly increased, and the safety injection is finished early (in the range from several tens of seconds to several minutes), so that the pressurized-type safety injection tank is not suitable for the integral reactor that must be actuated for many hours.
In addition, the active safety system has a difficulty in ensuring the reliability of a power supply system to actuate the above systems.